JPH10295670A - Method for forming many slice images to indicate object to be inspected - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form many slice images which represent an object to be inspected without prolonging the scanning time substantially. SOLUTION: A magnetic resonance imaging system executes scanning to collect the data for reconstruction of many slice images crossing. During one pulse sequence, each slice is selectively energized individually to generate crosswise magnetization, which is refocused collectively by a single non-selective refocusing pulse 220, and this 220 generates echo signals 226 and 228 for each slice image. The flip angle of energization pulses 208 and 216 is selected to such a value as removing the saturate line in the slice intersection part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The field of the invention is nuclear magnetic resonance imaging methods and systems. More specifically,
The present invention relates to collecting MRI data from multiple intersecting slices.

[0002]

2. Description of the Related Art When an object such as human tissue is exposed to a uniform magnetic field (polarizing magnetic field B ₀ ), the individual magnetic moments of spins in the tissue tend to align along this polarizing magnetic field, Precess around a magnetic field in a random state at the Larmor frequency specific to each spin. When an object, ie, tissue, is in the xy plane and is exposed to a magnetic field near the Larmor frequency (excitation field B ₁ ), the net alignment moment M _z rotates toward the xy plane, ie, a possible "tipped", as a result, generates a lateral (direction) the magnetic moment M _t net. Is a signal by the excited spins release, after stopping the excitation signal B _1, an image can be formed by processes which receives this signal.

When an image is formed using these signals,
The magnetic field gradient (G_x , G_y And G _z ) Is used
You. Typically, the area to be imaged
Is that the gradient described above depends on the particular localization method employed.
Therefore, a sequence of measurement cycles that
Has been scanned. The resulting set of received
Digitized and processed NMR signals
Image reconstruction using one of the well known reconstruction techniques
I do.

The present invention is described in detail in connection with one variation of the well-known Fourier transform (FT) imaging technique often referred to as "spin warp". The spin warp method is described in Physics in Medicine and Biology, 2nd edition.
Volume 5, pages 751 to 756 (1980) WA
"Spin Warp NMR Imaging and Its Application to Whole Body Imaging of Human Body" by Edelstein et al. ("Spin Warp N
MR Imaging and Applications to Human Whole-Body Im
aging "). This method uses phase-encoding magnetic field gradient pulses of various amplitudes prior to the acquisition of the NMR spin echo signal to obtain spatial information in the direction of this gradient. When implemented in two dimensions (2DFT), the spatial information in this direction is encoded, for example, by applying a phase encoding gradient (G _y ) along one direction, and then The spin echo signal is acquired in the presence of a readout magnetic field gradient (G _x ) in a direction orthogonal to the encoding direction, and the readout gradient present during the spin echo acquisition encodes spatial information in this orthogonal direction. A typical 2DF
In a T-pulse sequence, the magnitude of the phase encoding gradient pulse G _y is gradually increased in a series of views acquired during the scan (ΔG _y ), so that a set of NMR data can be reconstructed to reconstruct the entire image. Occurs.

[0005]

In magnetic resonance guided therapy, MR imaging is employed to assist in providing therapeutic energy to tissue. Such methods are described, for example, in U.S. Patent Nos. 5,526,814 and 5,443,068. Fast,
In addition to the need to create near real-time images, this application of MRI requires a maximum amount of visualization of the tissue in the affected space. Single slice high speed acquisition methods do not provide information about tissue outside the slice. This is disadvantageous for effective treatment. This is because the energy therefrom, whether from a laser, from focused ultrasound, or from cryotherapy, is spread over the tissue in a three-dimensional pattern. The spread of energy is determined by the energy absorption curve and the dynamic thermal properties of the tissue. All relevant parameters are heterogeneous and these parameters change during treatment. Therefore, to maximize the effect of the procedure and optimize patient safety, it is desirable to visualize as much of the energy distribution in the tissue as possible. However, when the coverage is expanded in this way, the time resolution is greatly reduced at the cost of the coverage, and this is unacceptable in applications where visual information must be obtained in a method close to real time.

In the past, images in which multiple slices were orthogonal were acquired by varying the slice selection gradient, phase encoding gradient, and readout gradient for each measurement pulse sequence. However, spins located in two or more slices are saturated at the intersection of the slices as a result of being exposed to the RF excitation pulse for both slices. The resulting image will show a normally black, saturated band along the area where that image slice intersects with one or more other image slices.

One technique for removing saturated bands is to select slices that intersect outside the region of interest. This approach allows two or more non-parallel slices to be imaged simultaneously. However, the requirement that each slice intersect outside the region of interest severely limits the planes that can be selected.

[0008]

SUMMARY OF THE INVENTION The present invention operates an MRI system to rapidly collect NMR data from a region of interest and reconstruct images of a plurality of intersecting slices within the region of interest. On how to make it. More specifically, a number of intersecting slices within the region of interest are sequentially excited with selective RF excitation pulses, each having a flip angle of less than 90 °, and the non-selective R
F refocusing (reconverging) pulse is applied,
The transverse magnetization for all of the intersecting slices is refocused, and NMR echo signals from successive slices are collected in the reverse order of excitation. The gist of the present invention is that the combined excitation at the crossing lines of the slice
The task is to select the flip angle of the RF excitation pulse to produce a signal strength that is substantially the same as that generated in the non-intersecting regions of the slice.

It is a general object of the present invention to collect NMR data for a large number of intersecting slices without substantially increasing the scan time. By selectively exciting each intersecting slice and then non-selectively refocusing the transverse magnetization of these slices, multiple spin echo signals are collected in each pulse sequence.

It is another object of the present invention to form an image in which the saturation line at the intersection of the slices is minimized. By properly selecting the flip angle for each RF excitation pulse, the saturation line can be eliminated. For example, if a 60 ° RF excitation pulse is used for two intersecting planes, the resulting flip angle at the intersection is 120 °. This produces substantially the same signal as for a flip angle of 60 °. This is because the signal strength is proportional to the sine of the flip angle, and sin60 ° = sin120 °.
In this way, the intersection line is iso-intense with the rest of the slice, eliminating the major drawback of simultaneous excitation of the intersecting slices. This principle is
It can be extended to excitation of three orthogonal slices. However, at the cost of combining excitation pulses,
At the intersection of all three planes producing a flip angle of 80 °, a signal dropout occurs at the center.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, FIG. The figure shows the main components of a preferred MRI system incorporating the present invention. The operation of the system is controlled from an operator console 100 that includes a keyboard and control panel 102 and a display 104. The console 100 communicates with the independent computer system 107 via the link 116, and the computer system 107
Allows an operator to control the formation and display of an image on the screen 104. The computer system 107 includes a number of modules communicating with each other via a backplane. These modules include an image processor module 106, a CPU module 108, and a memory module 113 known in the art as a frame buffer for storing image data arrays. The computer system 107
It is coupled to a disk storage 111 and a tape drive 112 for storing image data and programs and communicates with a separate system controller 122 via a high speed serial link 115.

The system control unit 122 includes a set of modules connected together by the backplane 118. These modules include a CPU module 119 and a pulse generator module 121, which is connected to the operator console 100 via a serial link 125. Via the link 125, the system controller 12
2 receives commands from the operator indicating the scanning sequence to be executed. The pulse generator module 121 operates the components of the system to perform a scan sequence according to the present invention. Module 1
21 generates data indicating the timing, intensity and shape of the RF pulse to be generated and the timing and length of the data acquisition window. The pulse generator module 121 is connected to a set of gradient amplifiers 127 and indicates the timing and shape of the gradient pulses generated during a scan. The pulse generator module 121 also receives patient data from a physiological data acquisition controller 129 that receives signals from a number of different sensors connected to the patient, such as electrocardiogram (ECG) signals from electrodes or respiratory signals from bellows. Receive. Pulse generator module 12
1 is connected to a scan room interface circuit 133, which receives signals from a therapy device (not shown) used to treat the patient. These signals synchronize the acquisition of NMR image data with the procedure being used. Via scan room interface circuit 133, patient positioning system 134 also receives instructions to move the patient to the desired location.

[0013] The gradient waveform generated by the pulse generator module 121 includes a G _x amplifier, a G _y amplifier, and a G _y amplifier.
Gradient amplifier system 127 composed of _z- amplifier
Is applied to Each gradient amplifier excites a corresponding gradient coil in the assembly, generally designated by reference numeral 139, to generate a magnetic field gradient used to encode the acquired signal for position. Gradient coil
The assembly 139 forms a part of the magnet assembly 141 and the magnet assembly 141
Is a polarization magnet 140 and a whole body type RF coil 152
And The transceiver module 150 in the system controller 122 generates pulses, and these pulses are:
The RF coil 152 is amplified by an RF amplifier 151 and transmitted / received (T / R) switch 154.
Is combined with The resulting signal emitted by the excitation nuclei in the patient can be sensed by the same RF coil 152 and coupled to a preamplifier 153 via a transmit / receive switch 154. The amplified NMR signal is demodulated, filtered and digitized in the receiver section of the transceiver 150. Transmission / reception switch 154
Is controlled by a signal from the pulse generator module 121 to electrically connect the RF amplifier 151 to the coil 152 during the transmission mode and electrically connect the preamplifier 153 to the coil 152 during the reception mode. I do. The transmit / receive switch 154 also allows the use of a separate RF coil in either the transmit mode or the receive mode.

NM captured by RF coil 152
The R signal is digitized by the transceiver module 150 and transferred to the memory module 160 in the system controller 122. When the scan is complete and the entire data array has been collected in memory module 160,
An array processor 161 operates to Fourier transform this data into an image data array. This image data is
Computer system 107 via serial link 115
And stored in the disk memory 111 here. In response to a command received from the operator console 100, the image data is transferred to the tape drive 112.
Or the image processor 106
Can be further processed and transmitted to the operator console 100 and also displayed on the display 104.

For more details on transceiver 150, see US Pat. Nos. 4,952,877 and 4,995.
No. 2,736, which are hereby incorporated by reference. FIG. 2 shows a preferred embodiment of the pulse sequence according to the invention. The scan performed using this pulse sequence collects NMR data that can reconstruct two images representing the spins of two perpendicular slices. These slices are shown in FIG. In the figure, the first slice 200 is located in the xy plane, and the second slice 202 is located in the xy plane. These two slices 2
00 and 202 intersect at reference numeral 204 to make this intersection 204 equal in the reconstructed image by proper selection of the flip angle used in the pulse sequence shown in FIG. Is the teaching of the present invention.

FIG. 2 will be described in detail. This pulse
The sequence is the respective slices 200 and 202
To separately excite the spins within. The first sly
The selective gradient pulse 206 is G_y Generated by the gradient
At the same time, the selective RF excitation pulse 208 is applied to
Transverse magnetization is generated in one slice 200. Known in the industry
, The frequency and bandwidth of the RF pulse 208 are
Place rice 200 properly along the y-axis and
The slice thickness is selected to excite. RF
The flip angle of the pulse 208 makes the intersection 204 equal in intensity.
It is the teaching of the present invention that it is selected to be maintained.
Thus, in this embodiment of the invention, the flip angle is set at 60 °.
Is defined. Then spin the rephasing gradient
Re-phasing by pulse 210 in conventional manner
And the transverse magnetization of slice 200 is G _z Generated by gradient
Phase encoded by the gradient pulse 212
You. As is well known in the art, the phase encoding
Luth 212 changes stepwise through a series of values during a scan
And sample the k-space along the z-axis.

[0017] Then, the second slice 202, a second slice select gradient pulses to be produced by the G _z gradient 2
It is excited by applying a voltage of 14. At the same time, a second RF excitation pulse 216 is applied,
The frequency and bandwidth of the excitation pulse 216 have been determined to select a slice 202 located at a desired location along the z-axis and having an appropriate slice thickness. The flip angle of the second RF excitation pulse 216 is also 60 °, resulting in two slices 200 and 202
The spins at the intersections 204 permutate by a total of 120 °. This produces an NMR signal of the same intensity as for a flip angle of 60 °.
The transverse magnetization of the second slice 202 is then phase encoded by a gradient pulse 218 generated by the G _y gradient.

The phase of the spin precession in both slices 200 and 202 is refocused by a non-selective RF refocusing pulse 220.
This refocusing pulse 220 is followed by crusher gradient pulses 222 and 224 before and after. The crusher gradient pulse 224 is the RF pulse 22
The phase of free induction decay occurring after 0 is shifted (dephased). Crusher gradient pulse 222 balances crusher 224 and maintains phase coherence within the slice. Crusher gradient pulse 22
2 includes a negative rephasing pulse that would normally follow the slice selection gradient pulse 214. RF refocusing pulse 220 is non-selective, and thus refocuses the spin phase in both slices 200 and 202.

As a result, the spins in the second slice 202 are refocused to produce the first echo signal 2
26, and the spins in the first slice 200 are refocused to generate a second echo signal 228. The first echo signal 226 is collected in the presence of a read gradient pulse 230 generated by a G _x gradient, and the second echo signal 228 is generated by a second read gradient pulse 232 also generated by a G _x gradient. Collected in the presence. NMR for the first slice 200
In order to properly sample k-space along the x-axis during data reading, the dephasing gradient pulse 23
4 has been generated using a G _x gradient, and a second dephasing gradient pulse 236 is provided to properly sample k-space along the x-axis during reading of NMR data for the second slice 202. Generated using a G _x gradient. The dephasing gradient pulse 236 is
One half of the gradient pulse 234 and one half of the readout gradients 230 and 232.

G_y The gradient pulse 238 has a phase encoding
Of the same size as the pulsing pulse 218 and
4,665,365.
Rewinding, that is, rephasing
Play a role. G _y The gradient pulse 240 is
2 remaining after reading out the echo signal 228 of FIG.
Dephase or "kill" the loose net transverse magnetization (ki
ll). G_z The gradient pulse 242 is used for the subsequent slice selection.
G_z Equal to one half of pulse 214, slice 200
The phase encoding gradient pulse 212
G_z Sliding so that all of the gradient pulses are offset
The coherence of the phase in the source 200 is maintained.

Those skilled in the art will recognize that two slices 200 and 2
It will be appreciated that by placing the intersection 204 of 02 at the target location of the therapeutic procedure, the results of the therapy may be monitored for two planes. Due to the relatively small flip angle used, the signal-to-noise ratio (SNR) of the image is slightly reduced, but more information is thus obtained with substantially no loss in time resolution. .

It will also be appreciated that the preferred embodiment can be modified without departing from the spirit of the invention.
For example, a third intersecting slice may be monitored. In such a case, a G _x slice selection gradient pulse is used with a third selective 60 ° RF excitation pulse. The resulting three images are of equal intensity except at a single location where all three slices intersect. At this single position, the spins are tilted by 180 °, resulting in a missing portion in the image.

[Brief description of the drawings]

FIG. 1 is a block diagram of an MRI system using the present invention.

FIG. 2 is a graphical illustration of a preferred pulse sequence executed by the MRI system of FIG. 1 to practice the present invention.

FIG. 3 is a sketch of two intersecting slices.

[Explanation of symbols]

REFERENCE SIGNS LIST 100 Operator console 102 Keyboard and control panel 104 Display 106 Image processing unit 107 Computer system 108, 119 CPU module 111 Disk storage unit 112 Tape drive unit 113, 160 Memory module 115 High-speed serial link 116 Link 118 Backplane 121 Pulse generation Module 122 System Control 125 Serial Link 127 Gradient Amplifier 129 Physiological Data Acquisition Controller 133 Scan Room Interface Circuit 134 Patient Positioning System 139 Gradient Coil Assembly 140 Polarizing Magnet 141 Magnet Assembly 150 Transceiver 151 RF Amplifier 152 Whole Body RF coil 153 Preamplifier 154 Transmit / receive (T / R) switch 16 Array processor 200 First slice 202 Second slice 204 Intersection 206, 214 Slice select gradient pulse 208, 216 Selective RF excitation pulse 210 Rephasing gradient pulse 212, 218 Phase encoding gradient pulse 220 Non-selective RF refocusing pulse 222,224 crusher gradient pulse 226 a first echo signal 228 the second echo signal 230, 232 readout gradient pulses 234 and 236 dephasing gradient pulses 238, 240 G _y gradient pulse 242 G _z gradient pulse

Claims (4)

[Claims] 1. A method for forming a plurality of slice images representing a subject in a plurality of intersecting planes within a region of interest using a magnetic resonance imaging system, comprising: Generating a corresponding series of slice selection field gradient pulses; and (b) simultaneously generating a corresponding series of selective RF excitation pulses to generate transverse magnetization in each of the plurality of planes, simultaneously with the slice selection field gradient pulses. (C) generating a phase encoding magnetic field gradient pulse for each of said plurality of planes; and (d) non-selecting to refocus transverse magnetization in all of said plurality of planes over a series of echo signal times. Target R
Generating an F refocusing pulse; (e) generating a readout magnetic field gradient pulse at each of the echo signal times; (f) collecting an echo signal for each of the plurality of planes; (g). Steps (a) to (f) until the echo signal is collected for a predetermined number of different phase encodings
And (h) reconstructing a slice image using the corresponding collected echo signals for each of the plurality of planes, wherein the flip angle of the RF excitation pulse is A number of slice images representing a subject whose intensity of the echo signal at the intersection of a number of planes is selected to be at a predetermined level with respect to the intensity of the echo signal throughout the number of planes are generated. How to form. 2. The flip angle of the RF excitation pulse such that the intensity of the echo signal at the intersection of the multiple planes is substantially the same as the intensity of the echo signal throughout the multiple planes. Claim 1 selected for
5. The method of forming a multiplicity of slice images representing a subject according to 1. 3. There are two intersecting planes, and said R for each of said two intersecting planes.
3. The method of claim 2, wherein the F excitation pulse has a flip angle of substantially 60 [deg.]. 4. There are three intersecting planes, and said R for each of said three intersecting planes.
3. The method of claim 2, wherein the F excitation pulse has a flip angle of substantially 60 [deg.].